GaN (gallium nitride) is one of Group III-V compound semiconductors and has a wurtzite crystal structure belonging to the hexagonal crystal system.
In recent years, GaN single crystal substrates have attracted attention as semiconductor substrates for nitride semiconductor devices.
Nitride semiconductors are also referred to as, for example, nitride-based group III-V compound semiconductors, group III nitride compound semiconductors, and GaN-based semiconductors, and include, in addition to GaN, a compound in which gallium in GaN is partially or fully substituted with another group 13 element (such as B, Al, and In) in the periodic table.
One of highly useful GaN single crystal substrates is a C-plane GaN substrate. The C-plane GaN substrate is a GaN single crystal substrate having a main surface parallel to or slightly tilted from the C-plane.
The C-plane GaN substrate has a gallium polar surface being a main surface on the [0001] side and a nitrogen polar surface being a main surface on the [000-1] side. So far, it is mainly the gallium polar surface that is used to form nitride semiconductor devices.
Cases where a C-plane GaN substrate was made from a GaN single crystal grown by an ammonothermal method have been reported (Non-Patent Document 1, Non-Patent Document 2).
In Patent Document 1, a GaN crystal was grown by an ammonothermal method on a C-plane GaN substrate provided with a pattern mask of a stripe type. NH4F (ammonium fluoride) was used singly as a mineralizer, and according to the description, a GaN crystal film having a flat top surface and a thickness of from 160 to 580 μm was grown through the pattern mask. It is unclear on which of the gallium polar surface and the nitrogen polar surface the pattern mask was formed.
In Patent Document 2, a GaN single crystal was grown by an ammonothermal method on a C-plane GaN substrate having a nitrogen polar surface on which a pattern mask of a stripe type was provided. NH4F and NH4I (ammonium iodide) were used in combination as mineralizers, and according to the description, after passing through the pattern mask, the GaN crystal further grew in the [000-1] direction without coalescence until the GaN crystal had a size of the order of millimeters in the C-axis direction.
Non-Patent Document 3 reported growth rates of GaN crystals obtained when various ammonium halide mineralizers were used in an ammonothermal method.